1. Field of the Invention
This invention relates to the detection of gaseous or vaporous ionizable chemical species in a photoionization detector, and in particular to a detection system in which the chemical species are detected using air as a carrier gas.
2. Description of the Prior Art
Photoionization detectors are often designed for use at the end of a gas chromatograph column and use radiation in the vacuum ultraviolet region of the spectrum to ionize chemical species borne into an ionization chamber from the column in a carrier gas. The photon energy of the radiation is at a level designed to ionize only the chemical species to be detected and not the carrier gas. Ionization is detected by electrodes in the ionization chamber electrically connected to an electrometer.
In gas chromatography, a carrier gas, which in the prior art for use in ionization detectors is generally an inert gas, flows continuously through a chromatograph column. A gaseous or liquid diluent, containing the ionizable chemical species under study, is introduced into the end of the chromatograph column remote from the detector. Component species elute through the column at different rates and are thus separated before reaching the detector. Such chemical species enter the ionization chamber of a photoionization detector in the carrier gas, become ionized, and give rise to a response from the electrometer. When displayed on a chart recorder each response appears as a peak whose arrival time depends upon the time taken for that particular contaminant to elute through the column before being ionized. By comparison with known standards the species can be identified and their quantity measured by determining the area under the peak displayed on the chart recorder.
The radiation used in a photoionization detector should be of high enough energy to ionize the chemical species to be detected, but not so high as to discernably ionize the carrier gas or any other species present which it is not desired to detect. Generally speaking the radiation used is ultraviolet radiation between 1000 .ANG. to 2000 .ANG.. Such radiation is of a low enough photon energy that it will not ionize any of the permanent air gases, such as oxygen, nitrogen, carbon dioxide, the inert gases or water vapour.
Such radiation is quickly absorbed in air and it will penetrate more than a few millimeters only in a vacuum or an atmosphere of inert gas. Thus it is commonly referred to as vacuum ultraviolet radiation.
The radiation source which is commonly used in photoionization detectors is a gas discharge tube made of glass and metal and filled with a suitable gas at low pressure. A crystal window composed of an appropriate transmissive material provides an exit for the vacuum ultraviolet radiation. In prior art, photoionization detectors have employed a discharge tube in which the discharge or excitation produced is by maintaining a high potential direct current across two metal electrodes located within the tube and in contact with the gas.
In a discharge tube of the above type, complicated tube designs have to be used in order to prevent electrode metal from being removed by ion impact in the process known as "sputtering". Such electrode metal is deposited upon the inner surface of the crystal window, thereby drastically reducing the transmission through said window and hence the operating life of the tube.
For example in Driscoll U.S. Pat. No. 3,933,432 issued Jan. 20, 1976, the high intensity portion of the gas discharge has been confined to a central capillary within the discharge tube by constraining the flow of ions as they move from one electrode to another. In this way, high ion current densities result in the central capillary but only very low ion current densities are established at the electrode surfaces. Where the effects of sputtering are controlled in this way, intensity and distribution of radiant flux must necessarily be heavily compromised, with resultant deterioration of the sensitivity of the detector.
The abovementioned design results in what is effectively a "point source" of vacuum UV radiation, originating from the small cross section of the capillary. As a result of this configuration, the distribution of radiative flux across the diameter of the ion chamber in any plane perpendicular to the direction of travel of the radiation entering the chamber is highly non-uniform, being high in the centre and low at the periphery. Apart from the reduced ionization due to limited total flux, such a design will exhibit strong "quenching" effects whenever a trace of oxygen is present in the chamber. Quenching occurs when an electron, generated as a result of photoionization, becomes attached to an oxygen atom, due to the high electron affinity of oxygen. The resulting negative ion has a mobility far lower than the original electron, and a far greater likelihood than an electron that it will recombine with a positively charged ion before it can be detected. Where a point source light is used a larger percentage of the oxygen ions migrate out of the field of ionizing radiation and recombine before they can be detected. Thus quenching will be a severe problem if a device utilizing such a source should be required to operate using air as carrier gas.
Discharge tubes have been built, for other applications, in which the metal electrodes are mounted external to an all-glass tube (fitted with an appropriate crystal window) and are connected to a supply of radio-frequency power. The radio frequency radiation is coupled capacitively into the gas and excites a discharge therein without the aforementioned sputtering effect. In prior art, such as Young U.S. Pat. No. 3,996,272, such tubes have been designed with one electrode being inserted into a hollow re-entrant capillary tunning up the axis of the cylindrical discharge cavity. The second electrode is formed from a metal cylinder, wrapped around the outside of the tube. The resulting coaxial electrode configuration functions as a capacitor. Tubes of this type are relatively difficult and expensive to make and, due to obstruction by the re-entrant capillary, do not have a radially uniform output intensity.